Many modern electronic applications require an ultra-stable frequency reference and/or an ultra-stable time reference for proper operation. For example, Global Positioning System (“GPS”) applications in general, and jam-resistant GPS receivers in particular, devices for wireless network time synchronization, for distributed network communications, and/or for distributed network position localization, and a host of military systems and platforms having sophisticated ultra-high frequency communication and/or navigation requirements all require such references.
Typically, in order to provide the necessary ultra-stable frequency and/or time reference an atomic clock is employed. As readily understood by skilled artisans, an atomic clock is an electronic timing device whose frequency is governed by the natural resonance frequencies of atoms or molecules of suitable elements. Although there are different types of known atomic clocks, the basic principle behind them uses the common property of atoms, set in a suitable environment, to absorb and to emit electromagnetic radiation at one frequency that is extremely stable over time.
The major differences relate to the element used and the means of detecting when the energy level changes. Some types of atomic clocks known in the art include cesium atomic clocks, hydrogen atomic clocks, and rubidium atomic clocks. Cesium clocks employ a beam of cesium atoms, in which cesium atoms of different energy levels are separated by a magnetic field. Hydrogen atomic clocks function in a similar manner, but they require a container with walls of a special material so that the atoms do not lose the high-energy state too rapidly. Rubidium clocks are the simplest and most compact of all atomic clocks and use a glass cell of rubidium gas that changes its light absorption when exposed to the proper microwave frequency.
For proper operation, an atomic clock's temperature needs to be precisely controlled. Preferably, an atomic clock should be kept substantially in thermal isolation, minimizing thermal conductance between components of the atomic clock system, to facilitate its stable operation. In addition, it is also important to provide vibration isolation of the clock, minimizing the relative displacements between its components under acceleration, in order to maintain the clock's mechanical stability.
Over the years, several approaches have been suggested for mounting, as well as for thermally isolating and/or controlling the temperature of an atomic clock. Some techniques employ a bridge made of thermally insulating material to suspend and isolate the clock cell from a substrate. The bridge is fabricated either from the substrate material itself, such as PYREX, or using a material deposited on the substrate, such as silicon nitride. A common feature of these approaches is that the bridge in its nominal configuration is in the plane, or parallel to the plane, of the substrate. Other techniques rely on thermally insulating posts extending from a substrate to support a clock cell and other clock components. In summary, many of known approaches utilize materials that are not of suitably low thermal conductivity and/or emissivity, materials that provide relatively poor heat-sensing sensitivity, and/or materials that are not mechanically robust, as well as suspension geometries that are not sufficiently stiff. Finally, known techniques typically rely on fabrication processes that are not amenable to low-cost parallel production.
Accordingly, there is a need in the art for an apparatus for thermally isolating electronic devices, such as, for example, atomic clocks, with improved mechanical stability and temperature control.